The present invention relates to a diamond coated body with a well adherent and wear resistant diamond coating, particularly a cutting tool aimed for cutting Al-alloys containing, e.g., Si and Cu as alloying elements and the like.
The production of thin coatings of diamond directly from the gas phase by CVD or PVD technique is of great interest for coating of cutting tools, drill bits, knives, etc. Gas phase deposition of diamond at useful deposition rates was reported in 1977 by Russian investigators. When the results were reproduced by a group of Japanese scientists in the early eighties, an intensive research in CVD diamond started all over the world. A series of new methods for diamond coating from gas phase has been described, including hot filament, microwave plasma, are discharge plasma, gas flame, hollow cathode, and different forms of plasma jets. Usually pure hydrogen with the addition of 0.1 to 5.0% CH.sub.4 is used as the gas mixture, but also other hydrocarbon gases can be used as the carbon source. In addition, the purity and quality of the diamond film can be controlled by adding other gases, for example oxygen containing and noble gases.
The main problem in applying CVD diamond coatings on cutting tool inserts (also referred to as substrates) for machining operations has turned out to be an insufficient adhesion of the diamond coating. The adhesion is strongly influenced by the surface characteristics of the insert. The main reason, however, for poor adhesion is believed to be the formation of voids and non-diamond carbon material at the coating/substrate interface in combination with high compressive stresses in the coating. The formation of high residual (compressive) stresses is partly due to the low coefficient of thermal expansion of diamond.
The substrates of interest for diamond coatings in cutting tool applications include cemented carbides and ceramics such as SiAlON and Si.sub.3 N.sub.4 and the deposition of diamond on these categories of substrates is well known in the art. However, as mentioned above, these prior art products suffer from an insufficient adhesion of the diamond coating as judged from machining tests. A suggested remedy for the poor adhesion is to apply an intermediate coating with properties that better adapt to the diamond than to the original substrate material. In JP-A-59-166671 and JP-A-59-166672, a method is described of coating cemented carbide or metal alloys with diamond and, before that, an inner intermediate coating of various carbides, nitrides, carbonitrides or oxycarbonitrides of the transition metals of the groups IV, V or VI of the Periodic Table, or of silicon, boron or aluminum. The possibility of having any of these types of coatings as the outermost layer is also mentioned. The coatings are deposited by magnetron sputtering. The tools are said to be excellent in wear resistance, however, the presence of an outermost coating (other than diamond) is not said to influence the wear resistance in the same manner as an intermediate coating does.
Another example where an intermediate coating is said to improve the adhesion between the diamond coating and the substrate is U.S. Pat. No. 4,734,339 (our reference: 024000-317). This patent describes a compound body which has a thin coating of metals with high affinity to carbon, certain carbides, nitrides and oxides or a noble metal between the diamond coating and the substrate. Again the possibility of having any of these coatings as the outermost is mentioned but not as being particularly favorable.
In Chattopadhyay et al, "On Surface Modification of Superabrasive Grits by CVD of Chromium", Annals of the CIRP, vol. 41/1/1992, pages 381-385, a method is described where diamond particles are coated with functioning layer of chromium or chromium carbide. In this paper, however, the adhesion between the chromium coated diamond particles and a surrounding matrix into which the particles are mixed, is in focus.